For decades, livestock producers have used low doses of antibiotics to expedite animal growth. The practice lowers feed costs while increasing meat production, and nearly 80 percent (you read that right!) of the antibiotics used in the United States are for this purpose. The evidence that low dose use of antibiotics in livestock encourages the growth of antibiotic-resistant “superbugs,” such as MRSA becomes stronger everyday. For that reason it’s banned in many countries, but remains common in the U.S. Although prophylactic antibiotic use in livestock has been in use since the 1950s, how it works has long been a mystery. But evidence is mounting that it might be due to antibiotics killing microorganisms that populate animals’ guts.

If so, antibiotics could do the same thing to humans. In support of this idea, a paper published last month in Nature, identifies a correlation between diversity of gut microflora and human obesity. A nine-year study, led by Dr. S. Dusko Ehrlich of France’s National Institute for Agricultural Research, compared microbiotas–the 100-trillion-member microbial ecosystems that populate the body–of slim and obese people.

The team found obese people have lower microbial diversity in their bellies. This is consistent with earlier research in mice, as well as a paper published last year in Journal of Obesity that found a strong correlation between young children’s exposure to antibiotics and later obesity.

Perhaps more significantly, the team behind the Nature study found a correlation between low microbial diversity and heart disease, diabetes and cancer, regardless of weight. “Even lean people who are poor in bacterial species have a higher risk of developing these pathologies,” Ehrlich told NPR.

All this supports the ideas that eating a poor diet or taking lots of antibiotics may be factors in the obesity epidemic and associated health problems, in part, because of the way they affect our gut microbes, Ehrlich says.